In a typical cellular wireless communication system, an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective base station. The base stations of the cells are then coupled to a switch or gateway (hereafter “switching system”) that provides connectivity with a transport network and/or to a signaling network. When a mobile station (i.e., wireless communication device), such as a cellular telephone, personal digital assistant, pager, or appropriately equipped portable computer, for instance, is positioned in a cell, the mobile station may then communicate via an RF air interface with the base station of the cell. Consequently, a communication path can be established between the mobile station and the network, via the air interface, the base station and the switching system. In this way, the base station(s) and switching system work in combination to function as a radio access network (RAN), providing mobile stations with RF access to engage in network communications, such as telephone calls or Internet communications.
In general, the air interface used for communications from a base station to mobile stations (i.e., the forward link) may be divided into a number of channels, including traffic channels used to carry bearer traffic (e.g., voice or other user data) and control channels used to carry overhead messages. Depending on the wireless technology used, the air interface can be divided into these channels through code division multiplexing (with each channel defined by modulation with a specific code), time division multiplexing (with each channel defined as a segment of time), frequency division multiplexing (with each channel defined by modulation with a specific frequency), and/or some other mechanism.
By way of example, in traditional Code Division Multiple Access (CDMA) systems, the forward link may define up to 64 channels, each distinguished by a unique “Walsh code.” The control channels include a pilot channel defined by Walsh code 0, a synch channel defined by Walsh code 32, and a number of paging channels defined by Walsh codes 1 through 7, as necessary. The traffic channels, in turn, are defined by the remaining Walsh codes (up to 62 in total). Further, in a CDMA system, each sector of a base station cell is distinguished by a PN offset, which defines a sector-specific part of a pseudo-random number. Communications between a base station and a mobile station on a given channel, in a given sector, and on a given carrier frequency, are encoded using the Walsh code of the channel and the PN offset of the sector and are then carried on the carrier frequency. A mobile station receiving such a communication can then extract particular channels from the air interface by employing a “rake receiver” that scans through air interface signals in search of a signals that are encoded with particular combinations of PN offset and Walsh code.
As another example, in systems operating according to the well known EV-DO protocol (as defined by industry standard IS-856 for instance), the forward link is divided into timeslots of 1.67 ms each, and each timeslot is further time division multiplexed to define various channels including a data channel and a control channel. The data channel is used to carry bearer data to a mobile station, and the control channel is used to carry control messages such as page messages for instance. In addition, as with legacy CDMA systems, each cell sector defined under IS-856 may have a respective PN offset and may operate on a particular carrier frequency, and so forward link communications may be encoded using the PN offset, modulated on the carrier frequency, and carried in a particular timeslot. Numerous other air interface protocols are known as well or will be developed in the future.
When a RAN receives a request to connect a call (e.g., a legacy voice call or VoIP call) to a mobile station, the RAN will typically page the mobile station in an effort to determine whether the mobile station is available to receive the call. In practice, for instance, the switching system may direct the serving base station to broadcast over an air interface paging channel a general page message directed to the mobile station. If the mobile station receives the page message, the mobile station would then respond with an acknowledgment message back to the RAN, which would cause the switching system to continue setup of the call to the mobile station. On the other hand, if the RAN does not receive an acknowledgement from the mobile station within a set period of time, the RAN may then re-page the mobile station. Further, the RAN may repeat the re-paging process a set number of times or until the RAN receives an acknowledgement from the mobile station. Ultimately, if the RAN does not receive any page-acknowledgement from the mobile station, the RAN may programmatically conclude that the call setup paging process failed. Consequently, the RAN may then responsively set up the call to a voice mail system, to allow the caller to leave a message for the called party.
Once the caller leaves a message at the voice mail system, the voice mail system will commonly send a message waiting indication (MWI) to the mobile station, to alert a user of the mobile station that a message is available for retrieval. In practice, the voice mail system may send the MWI as a data-over-signaling message to the RAN, in much the same way that network entities transfer short messaging system (SMS) messages from point to point. Upon receipt of the MWI at the RAN, the switching system may then direct the serving base station to transmit the MWI via the paging channel to the mobile station. Upon receipt of the MWI, the mobile station may then present a corresponding notification (e.g., an envelope icon, tone, vibration, etc.), to notify a user of the mobile station that a message is waiting to be retrieved.